Machinery Safety 101

Why you should stop using the term ‘Deadman’

The Deadman Control

Do you use the phrase ‘deadman’ or ‘deadman switch’ when talking about safety-related controls on your machinery? I often run into this when I’m working with clients who use the terms to refer to ‘enabling devices’ – you know, those two or three-position switches that are found on robot teaching pendants and in other applications to give the operator a way to stop machinery, even if they have already been injured or killed by the equipment. Calling these devices a ‘Deadman Switch’ or even a ‘Live-Man Switch’ as the three-position devices are sometimes called, sends entirely the wrong message to the user as far as I’m concerned. The objective of our work as machinery safety engineers is to prevent injuries from happening in the first place. Using a device that is designed to determine if the user is dead or unconscious means someone screwed up!

A little history

The term ‘deadman’ comes from a device that was developed in the 1880’s by pioneering electrical engineer Frank Sprague. Sprague was working on electric traction motor technology, using these new machines to power street railways (streetcars) and electric elevators. The early DC motor controls used in both streetcars and elevators required an operator. The operator used a hand control to move the streetcar forward or backward along the track and to control the speed of the car. In elevators, the operator used a similar hand control to move the elevator car up or down the shaft, to control the speed and to stop at the appropriate floor.

Westinghouse Streetcar Controller
1920’s era Westinghouse Streetcar Controller

If the operator was to doze off or fall unconscious, the streetcar would simply continue on its way until it hit something or derailed, either being a poor option! Elevators would continue until they hit the top or bottom of the shaft, again a bad idea. Sprague included a control device in his designs that required the operator to keep his hand on the controller handle and to maintain pressure on the control device in order for power to flow to the motor. This same idea was implemented in manual elevator control handles. These ideas were adopted by Westinghouse when they developed the streetcar motor controllers that were used in thousands of streetcars running between the 1890’s and the 1930’s.

When diesel-electric and full electric locomotives were developed in the 1930’s, the concept of the ‘deadman’ control was adopted from street railways. There is a persistent myth that these controls started with steam locomotives. In fact, an earlier version of this article included that myth – now busted!

A 'deadman' pedal in a locomotive.
A ‘deadman’ pedal in a diesel-electric railway locomotive

With the advent of electric trams, trains, and subways, concerns about possibilities like heart attacks and other infirmities resulting in drivers losing control of these machines caused these devices to be integrated into these new transportation systems. To learn more about these applications, see the Wikipedia article Dead Man’s Switch.

It’s worth noting that the railways now call these devices ‘Driver Safety Devices’ or DSD. See a modern DSD at the Arrowvale Electronics web site.

Elevators moved from manual control to automatic control, eliminating the need for elevator operators and the need for ‘deadman’ controls.

Robots Enter the Picture

Motoman robot pendant enabling device
Motoman pendant with showing enabling device (red arrow)

In the 1980’s, industrial robots began to appear in the workplace. Accidents in these early days drove changes in the design of the control pendants used to ‘teach’ these devices their tasks. Early pendants provided motion control and an emergency stop device. Later, the motion controls were altered to become ‘hold-to-run’ devices that could jog the selected robot axis at a pre-selected slow-speed, one axis at a time. In the 90’s the ‘enabling device’ was added to the pendant. These two-position switches, still called ‘dead-man switches’, had to be held closed in order for the robot to move under control of the axis hold-to-run controls. Accidents continued to occur. In the mid 90’s the three-position enabling device, sometimes called a ‘live-man-switch’, was introduced after studies showed that some people would release their grip on the control pendant when struck by the robot, while others would clench the hand holding the pendant. The new switches are required to be held in the mid position to enable motion. The picture at left shows the back of a modern robot pendant. The black bar in the lower right is the enabling device, located so that your hand will naturally hold the device in the correct position when you hold the pendant in your left hand. Not so good if you are left-handed!

ABB IRB640 Robot Pendant
ABB IRB640 Robot Pendant

 

 

Euchner ZS Switches

In addition to the pendant enabling devices, additional enabling devices are required where more than one worker is required inside the danger zone of the machine. These devices can be purchased separately and added to systems as needed. Depending on the application, you can get these devices with emergency stop buttons and jog buttons integrated into a single unit as shown in the picture of the Euchner ZS switches.

Machinery Standards and Definitions

The enabling device is one of those protective measures that cannot be readily classified as a safeguarding device because they do not proactively prevent injury. Instead, like an emergency stop, they may allow a worker to avert or limit the harm that is already occurring. That places the enabling device into the ‘complementary protective measure’ category.

Let’s take a minute to look at a couple of important definitions from the machinery standards. At the moment, the best definition for a complementary protective measure comes from the Canadian standard, CSA Z432-04. Excerpted from CSA Z432-04, §6.2.3.5.3 Complementary Protective Measures:

Protective measures that are neither inherently safe design measures, nor safeguarding (implementation of guards and/or protective devices), nor information for use may have to be implemented as required by the intended use and the reasonably foreseeable misuse of the machine. Such measures shall include, but not be limited to,

a) emergency stop;

b) means of rescue of trapped persons; and

c) means of energy isolation and dissipation.

Let’s also look at the formal definition of an ‘enabling device’ in the same standard:

7.23.3 Enabling devices
7.23.3.1
An enabling device is an additional manually operated 2- or 3-position control device used in conjunction with a start control and which, when continuously actuated in one position only, allows a machine to function. In any other position, motion is stopped or a start is prevented.

7.23.3.2
Enabling devices shall have the following features:

a) They shall be connected to a Category 0 or a Category 1 stop (see NFPA 79).

b) They shall be designed in accordance with ergonomic principles:

(i) position 1 is the off function of the switch (actuator is not operated);

(ii) position 2 is the enabling function (actuator is operated); and

(iii) position 3 (if used) is the off function of the switch (actuator is not operated past its mid position).

c) Three-position enabling devices shall be designed to require manual operation in order to reach position 3.

d) When returning from position 3 to position 2, the function shall not be enabled.

e) An enabling device shall automatically return to its off function when its actuator is not manually held in the enabling position.

Note: Tests have shown that human reaction to an emergency may be to release an object or to hold on tighter, thus compressing an enabling device. The ergonomic issues of sustained activation should be considered during design and installation of the enabling device.

 

OMRON A4EG Enabling Switches
OMRON A4EG Enabling Switches

Similar definitions exist in the International, European and US standards, although they may not be quite as formalized.

 

Most enabling devices on their own do nothing except PERMIT motion to take place, although the actual definition of enabling device in CSA Z432-04 actually permits the enabling device to cause motion. Absence of the enabling signal prevents or stops motion. These devices are then used in conjunction with hold-to-run controls on robots and machinery, and with throttle controls on trains, street cars, subways and similar equipment. Note that most standards to not permit enabling devices to actually cause motion. This is a unique situation in the Canadian standard.

So what’s the big deal?

Using the terms ‘dead-man’ or ‘live-man’ to describe these devices puts the wrong message out as far as I’m concerned. As safety engineers and OHS practitioners, we care about keeping workers out of danger. This is neither checking to see if we have either a ‘dead man’ or a ‘live man’, but rather ensuring that the person in control of the equipment is ‘in control’.  Using a phrase like ‘enabling device’ clearly says what the device does.

In my opinion, and  supported by the current International and Canadian Standards, these terms must be abandoned in favour of ‘enabling device’ and the qualifiers ‘2-position enabling device’ and ‘3-position enabling device’. These terms are also used in many of the current machinery safety standards, so using them correctly improves clarity in writing and speaking. Clarity in communication in safety is too important for practitioners to permit the ongoing use of terms that convey the wrong message and do not promote clarity of meaning. Since clarity is often lacking when it comes to safety, anything we can do to improve our communications should be high on our priority list!

Ed. note: This post was updated on 17-Aug-17. The myth of deadman controls on steam locomotives was removed and replaced by historically verifiable information about the origins of this control.

20 thoughts on “Why you should stop using the term ‘Deadman’

  1. I’m interested to find out who invented the deadman switch. I see from your article that Frank Sprague developed it, but I can’t find any resources to suggest he was associated with its design. Would you mind sharing this information at all?

    Many Thanks!

    1. Hi Max,

      I wish I could tell you more, but the best I could do in my research for this post was that Sprague invented it as part of the development of the electric trams and elevators that he was working on in the earliest years of the 20th century. His papers are held in an archive at the New York City Library, and I would love to spend a few days there to see what more I could learn. Unfortunately, until the pandemic-related travel restrictions lift, I don’t see a way to do that. It would be great if archives like this were available online, but so far that is a rare thing.

  2. Good timing! I was just at the Automate Show, working in the Robot Safety Standard booth. I was demonstrating some new robot capabilities, then said “I hold the enabling device in the center “on” position”. One of the observers chimed in “the deadman switch”. I carefully explained the history – good thing it was the same as the article 😉 And then I explained that a deadman switch was developed to detect a dead man and stop the train so that more people do not die. However the enabling is used to keep people from harm, have control of the system, and ENABLE the equipment to operate within parameters (reduced speed, reduced torque, limited time duration,…).

  3. I don’t agree in the slightest. Euphemisms have no place in H&S. Call the damn thing what it is instead of being too proud to admit there could be a cock-up that requires its use. Next thing you know we’ll be calling it by some silly abbreviation like PPE (we used to call that safety gear if you can remember that far back).
    A deadman situation is absolutely not the right occasion for going looking in newspeak dictionaries.
    “Enabling button” especially I find ridiculous. An on/off button enables. This is much more specific than that.
    Don’t change a single thing. If it’s already been changed by snowflake legislators, admit the mistake and revert it.

    1. Dave,

      Thanks for your reply. I can see your point, however, this is not about being politically correct or some other notion involving snowflakes. 🙂 Enabling devices have one purpose in life: to enable hazardous motion under enhanced safety conditions. This means “enhanced” as compared to not having the enabling device or any other safeguarding. The phrase “enabling device” accurately describes the function of the device: it enables hazardous motion. Using the term “deadman” indicates that you’ve already failed and the worker is now dead. You just want to stop the machinery mangling the corpse too much before you can extract it for the family. THAT to me is unacceptable terminology.

      In any case, change already happened. The term “enabling device” is firmly embedded in all of the relevant standards and has been for quite a few years. Rooting it out will likely be very difficult to do, especially to replace it with what is now considered outdated language.

      1. We’ll agree to differ then.
        What is the purpose of a deadman switch? It’s not to stop a corpse being mangled. It’s to stop the train running away and gatecrashing a level crossing or whatever. The operator is disabled so you want the machine to stop because nobody leaves uncontrolled machinery running. Call it a fail-safe or sign-of-life switch if you prefer, but make it mean the same as its function, which is to stop by default, not to enable.

        I actually stumbled on this blog because I was searching for the most common usage: deadman switch? button? dead man’s handle?
        See I’m translating from French to English, and guess what it’s called in French parlance.

        1. Dave,

          Interesting. Are you reading my blog in French? Since I write in English and am a native English speaker I am interested.

          The usage of the term “deadman” in the rail sector is different than in the machinery sector. You are correct about the usage in rail locomotives and light rail vehicles like surface rail subways and streetcars. These are machinery, but they are subject to predominantly manual control. They are also generally not subject to the types of safeguarding systems that are commonly used to reduce risk in the machinery sector. By the way, sorry for the reference to mangled corpses, but I felt the need to stop the trend of the conversation. And I have to disagree – it’s quite possible to have a person entangled in a running machine for an extended period of time. See (WARNING: Graphic!) https://www.liveleak.com/view?i=bc3_1481760085 or https://www.liveleak.com/view?i=c5a_1494366041 for examples. Enabling devices might have helped in these circumstances.

          In the machinery sector, it is very common to have an individual worker take on a task in a relatively isolated part of a factory. An example might be programming a robot to do a task. The technician is normally inside the danger zone, often hidden from view, and working on potentially lethal equipment. If something goes wrong, the worker is right in the line of fire and is very likely to be injured or killed. There are no other safeguards other than a hold-to-run control, and an enabling device. These are complemented by an emergency stop button, but honestly, few workers hurt in these conditions have the thought to press that button, nor do they have sufficient time. So the enabling devices are designed to switch off power if compressed or released, based on studies that showed that some people will grasp something in their hands with all their strength when being injured, while others will drop it under the same conditions. This is a completely different scenario than having a locomotive engineer fall asleep, or become unconscious while the train is in motion.

          So, just to wrap this up, machinery people call them enabling devices. Rail people call them “deadman pedals” or “sign-of-life” devices. I am interested to learn more about your interest in this topic, and I would be more than happy to carry on the conversation via email or Skype as you like.

          1. Doug, I’m not sure what you mean about reading your blog in French but I can assure you all the words I see are in English.

            I translate from French to English for a living and in this case the subject was a torpedo handling system (I don’t think I’m giving any military secrets away saying that!). The thing is, although the source language is French they’ve already gone ahead put English labels on the hand held controller that houses this button, and the word they’ve put on the eponymous label is a great big “DEAD MAN”. But I still had to translate the description.

            They do use the word-for-word equivalent term “Homme mort” in French.
            https://fr.wikipedia.org/wiki/Protection_du_travailleur_isol%C3%A9#Type_d’alarme
            They also have a term “signe-de-vie” (sign of life), but that requires positive action from the operator every x seconds to keep the thing running, in which case “enabler” would be a valid if less evocative description. And I think that’s probably the point I’m making. To me it seems something much more evocative than “enabler”, which doesn’t give instant context, is called for.

            What, I’m wondering, do you call devices where you have to keep the button pressed in order to keep both hands busy, for example on the tailgate of a delivery van?

            Anyway, I digress. In the torpedo example, since the device is located on a wired remote control unit specifically designed to enable the operator to get far enough away for an overall view, they aren’t going to be anywhere near the machinery (namely an inclined winch) when using it. The only purpose I can really see for it is to stop the torpedo running out of control down into the submarine if the operator faints or something, which puts it closer to the runaway train category I suppose.

            That was an interesting point about more sophisticated devices reacting to the “grip of panic” as well as releasing. I wasn’t aware of that, and obviously that’s a much different situation from those I was referring to. I’ll bear it in mind if it ever comes my way.

            Thanks for the discussion

            Dave

          2. Dave, sorry, I misunderstood what you meant when you said you were translating from French into English – I thought you meant the blog! ? Oops! Anyway, I can definitely see where your comments are coming from. In the case of the torpedo hoist, “deadman” might very well be appropriate, considering what might happen to a submerged boat if a torpedo got away from the loaders while it was still inside the boat! I’ve heard rumours that the Russian submarine Kursk may have been lost through just such an accident.

            I agree about the sign-of-life devices. These are very common on rail locomotives and came about because there were a few cases where an engineer put his toolbox down on the “deadman” pedal so he didn’t have to maintain pressure on the pedal. There were predictable results from those escapades as you might imagine. I think what we are running into here is differences in usage based on the sector in which similar, but different, control functions are used. When it comes down to this, at least from an engineering perspective, this is a safety control function that has a specific outcome – stopping unattended or out-of-control hazardous motion.

            The tailgate devices you mention are usually called “two-hand control devices” because they keep the operator’s hands out of the works. These devices are also used on power presses, press brakes, and many other applications. There are some very detailed requirements for how these protective devices are designed and used. For more on that see ISO 13851.

  4. @dougnix:disqus Appreciated your work. I google the dead man switch, found your blog. Although English is not my first language however, I found it very informative and you are right the dead-man switch shall be change to enabling device or hold to run button.

    1. Ejaz Ahmad, thank you. I am glad to hear you found it helpful. Did you notice the translation widget at the top of the sidebar? You should be able to translate the article into your home language. If you try this, please let me know if it works well or not. 🙂

  5. The problem with his analysis is that, basically, he is looking at it from the operator view, inferring that operation or inoperation of the machine only impacts the operator, while the original purpose and its application in the contemporary disaster was to protect others from the machine when the operator did not show positive control (i.e. “dead”). There really are two different problems, possibly warrant two different names, but “dead man” is certainly appropriate for the problem that originated the term.

    1. Ken29,

      Thanks for your comment. I don’t disagree entirely – the original purpose was to prevent a runaway machine from injuring others and destroying itself. The death or disability of the operator was ancillary to protecting the machine and others who might be affected.

      Having said that, the term “deadman” is no longer used in technical standards or in regulations related to safety. These devices have had other names coined for them, dependant on the sector. As I mention in the article, the railways now call these systems out as “Driver Safety Devices”, while in automation and robotics they are termed “enabling devices”. In my opinion, both terms are more descriptive of the function of the device or system than the old “deadman”.

      1. Doug: I certainly agree that “deadman” by itself isn’t particularly informative and “deadman switch” only slightly better, but anything that ends in “device” is pretty ambiguous and probably more suited to standards and legislation than to popular vernacular. Not that I have thought of a better alternative.

        1. ken29,

          I see your point, however the use of the term device allows for new technologies that may not be accomodated by “switch”. If you consider some of the older devices that have served this purpose, valves and switches have both served this purpose. Current standards reflect the change to 3-position devices that incorporate multiple switches and require the user to hold the device in a mid position. This device is designed to ensure that no what reaction a user has when injured, the device will remove power from the machinery. Language needs to advance as well as technology. It’s my opinion that this change, now more than 10 years old, reflects that advancement.

          1. Again, that’s great for techies, standards, and legislation, but not useful or helpful for everyday communication with the public.

          2. No argument. This is always the challenge. How can we most effectively convey important ideas to lay people, while avoiding jargon? At the same time, how can we effectively communicate complex ideas succinctly when dealing with other professionals.

            What I try to do with this blog is to de-mystify complex ideas. My goal is to try to help anyone who is interested in understanding the key ideas related to safety of machinery. There are a lot of misconceptions and confusion out there – I run into it in my practice all the time. If professional people working in this sector are confused, I’m sure non-professionals are too.

            So what’s the alternative in this case? “Deadman” has been widely used in the past, although it brings with it some ideas that may not be completely correct. “Enabling Device” says what the thing does, but is not as well understood. Do you think a third option might exist?

          3. Doug: This isn’t a completely satisfactory, or even satisfying, answer, but it seems to come as close as I’ve been able to devise. How about using “control” instead of device, then adding a word or two (as we usually do) to select the specific control?
            I spent some time Googling “device”, “switch”, and “control” to see how they were represented in Google (sort of contemporary vernacular) and control seemed to “win” by some margin. I think we got into this conversation through the train crash out East and for that application I think a “deadman control” works fairly well and improves on “deadman switch”. I saw three-function “grip” used on an industrial machine and that matched the picture well, but I think control would work as well.
            Good luck! It’s a worthy cause.

          4. Ken,

            I agree, “control” covers it effectively and could be used to include any kind of “device” or devices necessary for any type of application. Good idea!

            Modern railway Driver Safety Devices (DSDs) are more complex than the pedal shown in the picture in the article. That one is pretty old-school. I’ve heard from a few people that the pedal shown in the article often ended up with a toolbox holding it down, defeating the purpose of the control.

            Now they usually require the driver to actively do something within a certain period of time, like press and release a button, or move a control, etc. Every application has it’s own unique requirements based on what the operator is required to do to use the machine.

  6. Good timing! I was just at the Automate Show, working in the Robot Safety Standard booth. I was demonstrating some new robot capabilities, then said “I hold the enabling device in the center “on” position”. One of the observers chimed in “the deadman switch”. I carefully explained the history – good thing it was the same as the article 😉 And then I explained that a deadman switch was developed to detect a dead man and stop the train so that more people do not die. However the enabling is used to keep people from harm, have control of the system, and ENABLE the equipment to operate within parameters (reduced speed, reduced torque, limited time duration,…).

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